Download Measuring Crest Factor

Application Note
Measuring Crest Factor
Yiu-Fai Kwan
Microwave Engineer, Noisecom
Abstract
This document provides Noise Com’s method of measuring crest factor of a noise
source using LeCroy Wavepro 7200 Oscilloscope.
Measuring Crest Factor
Crest Factor
Procedure
The Crest Factor (CF) is defined as the ratio of the peak amplitude
of a waveform to its RMS value. It is a numerical value without
units.
The LeCroy Wavepro 7200 Oscilloscope is used for measuring crest
factor. This is a four channel oscilloscope, any one channel can be
used to perform the measurement. For the following instructions,
channel 3 was chosen.
1. Turn the touch screen on.
CF is the Crest Factor
Vp is the peak voltage
Vrms is the RMS voltage
2. Press the “3“ button under “VERTICAL”.
3. Press the “C3” box on screen (on lower left hand corner).
Statistical evaluation of related occurring events present always
certain kinds of distribution curves. Gaussian distribution, also
known as normal distribution, is the most common way events
take place. Most white noise sources provide Gaussian distribution
of their spectral power. The randomness of noise sources requires
a mathematical model that allows calculating the probability
density of events. Gaussian distribution right and left of the
mean probability is of equal size. Crest factor is also an important
quantity for noise sources. It allows calculating the likelihood of
unwanted events relative to the mean probability. Such renegade
events may cause an overload of an amplifier input, distortion of
a signal, or a bit error. Obviously, the fewer of those natural but
unwanted events happen, the better. High quality white Gaussian
noise sources offer crest factors of up to 18 dB, placing the probability of unwanted events far beyond reasonable measurement
capabilities.
Application Note
4. Set “Trace on” with a check mark and set the following
parameters to:
Volt/div = 1.00 V
Offset = 0.00 V
Bandwidth = full
Coupling = DC50 ohm
Averaging = 1 sweep
Interpolation = Linear
Deskew = 0
Probe Atten = divided by 1
Measuring Crest Factor
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Figure 1 shows the Oscillscope display.
5. Next, press the “Timebase” box to
adjust the following
parameters:
Time/div = 5.00 us
Max sample Points = 500 kS
Active Channel = 10GS/s
Figure 1
Figure 2 shows the Oscillscope display.
6. Press “Math” under “Wavepilot “ on
the panel.
7. On the touch screen, press “F1” and set
a check mark to “Trace on”.
Figure 2
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8. Set “Source 1” to “C3” and “Operator
1” to “ Histogram” (see Figure 3).
9. At the right side box on screen, press
“Histogram” tab.
Figure 3
10. Set “Buffer” value to max (2 billion)
and “ # Bins” to max (2.00e+3)
(see Figure 4).
11. Press “Clear Sweep” on the lower
right corner panel. Now, the screen
should have two curves. One is the
output and the other one is the histogram of the output.
12. Press “Measure” under “Wavepilot”.
13. Set check mark on “Show Table” and
“Statistics”.
14. Press “P1” tab and set “Source 1”
to “C3”.
Figure 4
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Measuring Crest Factor
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15. Set “Measure” to “Maximum”
(see Figure 5).
16. Press “P2” and set “Source 1” to
“C3”.
Figure 5
17. Set “Measure” to “RMS”
(see Figure 6).
18. Press “P3” and press “Math on
Parameters”.
Figure 6
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19. Choose the “Math Operator” to “Pn /
Pm” (see Figure 7).
20. Make sure the “ON” box of P1, P2, &
P3 has a check mark.
21. The max value of P3 is the crest factor ratio, which is the max of P1 divided by the mean of P2. Take the log
of the max value of P3 and multiplied
by 20. That would be the crest factor
of the noise module in dB. In this
case, the crest factor is 16.39 dB.
Figure 7
Wireless Telecom Group Inc.
25 Eastmans Rd
Parsippany, NJ
United States
Tel:
+1 973 386 9696
Fax:
+1 973 386 9191
www.noisecom.com
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are subject to change without prior notice.